mockchain.log (global backend log) should contain info about signing rpms and calling createrepo, e.g.:

[2018-01-08 08:57:43,526][ INFO][backend.worker-9564-PC.builder.mr][init.py:do_createrepo:227] Createrepo:: owner: amahdal; project: slop; front url: https://copr.fedorainfracloud.org; path: /var/lib/copr/public_html/results/amahdal/slop/fedora-27-x86_64; base_url: https://copr-be.cloud.fedoraproject.org/results/amahdal/slop/fedora-27-x86_64

Seems that it is missing, e.g. here: https://copr-be.cloud.fedoraproject.org/results/amahdal/slop/fedora-27-x86_64/00697389-slop/mockchain.log.gz

Also rename the file to backend.log. mockchain.log is an outdated name from times when mockchain was used for building.